Recovery of isoprene from hydrocarbon mixtures comprising same and piperylene



may satisfactorily Patented Aug; 7, 1945 RECOVERY OF ISOPRENE FROM .HYDRO- CARBON MIXTURES COMPRISING SAME I AND Stephen C. Stow e, Midland, Mich., assignor tolhe I Dow Chemical Company, Midland, Mich, a cor-" poration of Michigan No Drawing. Application December 4, 1942 I Serial N0. 467,899

I 1 Claim. romeo-681.5) I

' invention concerns-a method of recovering isoprene of good purity'from hydrocarbon mixtures comprising the same and its isomer, piperylene. I t particularly concerns the recovery of substantially pure isoprene from a mixture of the sulphones of isoprene and piperylene.

' In industrialprocesses for the pyrolysis of petroleum fractions and other hydrocarbon starting materials, low'boiling mixtures comprising conjugated diolefines are produced in large quantities. The mixturesthus produced are usually complex; For instance, cracked-oil gas, a readily the sulphones of isoprene and piperylene,such as. may be obtained by treating a mixture comprising said diolefines with sulphur dioxide, is heated,

, both sulphones undergo. decomposition at the same temperatures; but that the rate of decom I position of'thesulphone of piperylene far exceeds that'of the sulphone of isoprene. By heating the mixture of isomeric sulphones at a moderate reaction temperature, the sulphone of piperylene may be decomposed to evolve sulphurdioxide and available source of diolefines, usually comprisesparaflinic hydrocarbons ranging from methane to hexane, olefines'ranging from ethylene to hexylene, diolefinessuch as butadiene, isoprene, piperylene, etc'., and acetylenic hydrocarbons su'ch a's" acetylene, methyl-acetylene, ethyl-acetylene, and vinyl-acetylene, etc.

I Such complex hydrocarbon mixture may be fractionally distilled to obtain fractions consisting for the most part of hydrocarbons having the same number of carbon atoms in the molecule,

carbons such as butan'es, butylenes, butadiene, and acetylenic hydrocarbons containing from 3 to i 4 carbonatoms in. the molecule and a C5 fraction consisting largelyof pentanes, amylenes, the conjugated diolefines isoprene and piperlylene, and acetylenic hydrocarbons. Way are known for separating conjugated diolefines from other kinds of hydrocarbons, and butadiene, which is the only "conjugated diolefine in "the C4 fraction, may be isolated from the Crfr'action in known ways. However, no method is known whereby either of the individual diolefines, isoprene and piperylene, v be recovered in pure form from 'thefCz. fraction;

-"-It is" an object of this invention to provide a practical method whereby the major portion of the isoprene in a hydrocarbonmixture comprising the same andpiperylenemay readily be separated insubstantially pure form.

It is-reported in'the-literature that the crystal-- line monomericsulphones of butadiene and isoprene-'eachdecompose at 120 C. to form a gaseous 'mixture of sulphur dioxide and the diolefine. I have observed thatthis teaching is not correct and'that the decomposition may occur at con-; "side'rably. lower temperatures; A reduction in the pressure on the system as a whole, or a reduc- "tionin-"thepartial pressure of'either reaction product, has the effect of increasing the rate of decomposition. v I

' I have .further foundthat when a mixture of piperylene, together with some isoprene, leaving the major portion of the sulphone of isoprene substantially pure. The residual sulphone of isoa prene may thereafter be'thermally decomposed to recover the isoprene in a form substantially free -e. g. .a C4 fraction consisting largely of hydroof other hydrocarbons.

In practice of the invention, a hydrocarbon mixture comprising isoprene and piperylene. is reacted inliquid phase with sulphur dioxide to form the sulphones of the diolefines. Any hydrocarbons other than conjugated-diolefines, e. g. parafiinic hydrocarbons, olefines or acetylenes such as pentanes, amylenes, or pentynes, etc., remain unreacted ,andarereadily separated from thesulphone products. Procedures for carrying out the-reaction to form sulphones of conjugated diolefines and for separating the sulphone products from unreacted hydrocarbons and sulphur dioxide are well known and do, not require detailed description. Cracked-oil gas usually contains small amounts of isoprene. andpiperylene,

each in a relative proportion corresponding to at least 20 per cent of the combined weight of the two. compounds, and the sulphones are, of course, obtained in similar proportions.

The mixture of the sulphone productsis heated sufliciently to cause a smooth and fairly rapid, but

notsudden, decomposition of the sulphones with. evolution of the resultant mixture of sulphur dioxide, piperylene and isoprene, whereby the sulphoneof piperylene is decomposed far more rapid.- lythan is that of isoprene and is removed before more than a minor amount of the sulphone of isoprene is destroyed. It is probable that the sulphone of piperylene decomposes more rapidly than that of isoprene regardless of the temperature employed and the step of decomposing and removing the sulphone of piperylene to leave the major portion of the sulphone of isoprene in purified form may be carried out at temperatures as high as C., or possibly even higher. ;I-Iowever, withdrawal of the piperylene from the reaction mixture before an excessive amount of the sulphone of isoprene is decomposed becomes more difficult as the reaction temperature is raised.

carried out at temperatures below 120 C., and

preferably between 70 and 115C.

The decomposition may be carried out in the presence or absence of solvents or liquid diluents" such as water, kerosene, or benzene, etc., and at any desiredpressure, e. g. at atmospheric, superatmospheric or at subatmospheric pressure, and

the temperature at which the decomposition may most advantageously be carried out is dependent in part upon the other conditions'employed. By carrying the decomposition out in the presence of a diluent or by reducing the pressure on the system the rate of decomposition is increased and satisfactorily rapid decomposition may be obtained at a temperature lower than would otherwise be required.

In practice, the mixture of sulphones is usually heated to a temperature sufficient to cause a fairly rapid evolution of the decomposition products and the temperature is gradually raised as necessary for continued reaction at a satisfactory rate. Operation in this manner iscontinued until substantially all of the sulphone of piperylene has been removed. The residual sulphone of isoprene is thereafter decomposed in the usual manner, e. g. by heating at 120 C. or above, to obtain a mixture of sulphur dioxide and isoprene. This mixture is scrubbed with water, or a dilute aqueous alkali solution, or treated in other usual ways to separate the sulphur dioxide and isoprene, whereby the latter is obtained in purified form. The yield of purified isoprene from the foregoing steps depends to some extent, of course, upon the pro-' portion of isoprene in the initial hydrocarbon fraction, but is usually more than 50 per cent of theoretical.

The yield may be improved by collecting the mixture of sulphur dioxide, piperylene and isoprene which is evolved during the stepof thermal- 1y decomposing the sulphone ofpiperylene, treating the mixture to remove the sulphur dioxide,

and fractionally distilling the remaining mixture v of piperylene and isoprene to concentrate the latter. Although, it is difficult, if not impossible, to separate either isoprene or piperylene from a cracked-oil gas fraction by fractional distillation, a mixture of these diolefines alone, which contains only a small amount of isoprene, may readily be distilled to obtain a first fraction of distillate containing the major part of the isoprene in a relatively concentrated form, e'. g. in a concentration of per cent by weightor higher. This fraction maybe returned to the step for the formation of the sulphones of isoprene and piperylene and the mixture of sulphones maybe treated as hereinbefore described to again recover the major portion of the isoprene in purified form.

The process as just described may be practiced in batchwise or in continuous manner, as desired. By continuously feeding a hydrocarbon mixture comprising isoprene and piperylene to the process and concentrating and recycling the isoprene evolved during the step'of selectively decomposing the sulphone of piperylene, 90 per cent or more'of the isoprene may be recovered as the substantiallypure compound. The following example describes one way in which the invention has been practiced, "but is not to be construed as limiting the invention.

Example prepared byrtreating .a liquefied cracked-oil gas leased and the mixture was gradually warmed to 0., whereby the excess sulphur dioxide and unreacted hydrocarbons were vaporized from the sulphone products. Thereafter, the temperature was gradually raised. Decomposition of the sulphones become noticeable at C. and occurred smoothly and quite rapidly at C. The temperature was raised further as necessary in order to maintain a fairly uniform evolution of vapors of the decomposition products, i. e. sulphur dioxide, piperylene and isoprene. The products were collected in successive fractions and each fraction was freed of sulphur dioxide by extraction with a dilute aqueous sodium hydroxide solution. The remaining hydrocarbons, which were in liquefied form, are referred to in the following table as the hydrocarbon condensate. Each hydrocarbon condensate was analyzed by a fractional distillation method to determine the proportions of isoprene and piperylene therein. The following table gives the decomposition temperatures at which the successive fractions of the products were formed. It also gives the volume of thehydrocarbon condensate obtained by removing the sulphur dioxide from each fraction and the -composition of each hydrocarbon condensate in terms of the per cent by volume of isoprene, of piperylene and of residue obtained or identified in the distillation.

It is probable that the residues which were obtain'edwhen the hydrocarbon condensates were distilled consisted in part of the diolefines held up as inventory in the distilling column and in part of polymers of the diolefines, and that the diolefine-content of the hydrocarbon condensates is somewhat higher than indicated in the above table. For instanceupon distillation of the hydrocarbon condensates corresponding to fractions 6, 7, and 8 there were obtained '75 c. c., 94.5 c. c., and 119 c. 0., respectively, of isoprene of highpurity. It is probable that the residues, in these instances, comprised isoprene retained in v thedistilling column and a smallamount of polymerized isoprene. Approximately 65 per cent of the isoprene initially present in the hydrocarbon .mixture was recovered as the pure compound.

od herein disclosed, provided the step or steps stated by the following claim or the equivalent of such stated step or steps be employed.

' I therefore particularly point out and distinctly claim as my invention:

In a method for separating isoprene from a hydrocarbon mixture comprising isoprene and V piperlylene, the steps of treating the mixture in v liquid phase with sulphur dioxide toform sul- "phones of the isoprene and piperlyene, vaporizing unreacted hydrocarbons and sulphur dioxide from'the sulphones, heating the mixture of $111- phones to a decomposition temperature between 70 and 120 0., whereby the sulphones are'gradually decomposed with evolution'of a vapor mixand 120 C., until the vapors being evolved are substantially free of piperylene and a substantial portion of the sulphone of isoprene remains, freeture of sulphur dioxide,, piperylene and isoprene, continuing heating at temperatures between'70' ing the evolved mixture of sulphur dioxide, distilling/the resultant mixture of piperylene and isoprene which remains after removal of the sulphur dioxide to obtain a fraction of distillate containing the major portion of the isoprene in relatively concentrated form together with some piperylene, recycling this fraction through the foregoing steps for the formation of the sulphones and forthe' decomposition of the sulphone of.

separating the sulphur dioxide and isoprene to recover the latter as the purified compound.

' STEPHEN C. STOWE. 

